Electrification is the most viable way to achieve clean and efficient transportation that is crucial to the sustainable development of the entire world. In the near future, electric vehicles (EV), including hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and pure battery electric vehicles (BEVs) will dominate the clean vehicle market. By 2020, it is expected that more than half of new vehicle sales will likely be EV models. The key and the enabling technology to this revolutionary change in transportation is the battery. EV batteries are quite different from those used in consumer electronic devices, such as laptops and cellphones. They are required to handle high power (up to a hundred kW) and have high energy capacity (up to tens of kW) within a limited space and weight and at an affordable price. The current two major battery types used in EVs today are nickel metal hydride (NiMH) and lithium type. Nearly all HEVs available in the market today use NiMH batteries because of its mature technology. Due to the potential of obtaining higher specific energy and energy density, the adoption of lithium secondary batteries is expected to grow fast in EVs, particularly in PHEVs and BEVs.
Laminated type lithium secondary batteries for both EV and HEV applications feature a structure in which stacked cathodes and anodes are alternately stacked with a separator sandwiched between and then sealed with a laminate film. The batteries are able to achieve a large capacity because of having an extremely compact shape. In addition, because of the simple structure, the batteries are lightweight and maintain a competitive advantage from a cost perspective as well.
A laminated type lithium secondary battery that boasts of advanced heat dissipation as compared to conventional cylindrical batteries. Because the laminated type lithium secondary battery has a broad surface area, the battery is better able to dissipate heat, and increases in the overall temperature of the battery due to charging and discharging can be kept low. Therefore, electric vehicles that adopt batteries of laminated type can simplify countermeasures against heat.
Cylindrical lithium secondary batteries (also known as wound type batteries), have an anode and a cathode which are cut into two long strips, and together with a separator, that keeps the anode and cathode apart, are wound on a cylindrical mandrel, to form a jelly roll (also known as a Swiss roll in the United Kingdom). Cylindrical battery thus has only two electrode strips which simplifies the construction of the battery considerably. The cylindrical design has good cycling ability, offers a long calendar life and is economical, but is heavy and has low packaging density due to it space cavities. The cylindrical cell is commonly used for portable applications.
Essentially, high elongation copper foil is more appropriate as a current collector in cylindrical batteries. When a cylindrical battery expands during its charge and discharge, the outermost circle of copper foil will expand more than the innermost (or inner) circles. If the copper foil does not possess high elongation, the outermost circle of copper foil will easily fracture.
Heretofore, a copper foil was used as a current collector of negative electrode (anode) in rechargeable lithium secondary batteries. The surface of the copper foil was coated with a layer of anode active material. Because the layer of anode active material expands and shrinks as it stores and releases lithium-ion, a stress is engendered in the current collector (copper foil) during a charge-discharge cycle and occasionally causes formation of wrinkles. The formation of wrinkles in the copper foil not only increases a volume of the negative electrode (anode) but also disturbs uniformity of an electrode reaction, resulting in a reduction of an energy density.
For laminated type lithium secondary battery, the expansion of the copper foil in X or Y direction of a laminated battery is not as much as in the cylindrical lithium secondary battery. Thus, the art has tended to use high tensile strength copper foils as a more appropriate current collector in laminated type lithium secondary battery. When a copper foil has high tensile strength, the copper foil has a high strength, it is more difficult to deform and cause wrinkles in the copper foil during the charge/discharge cycle of the battery.
In order to have high energy capacity, the thickness of the copper foil needs to be decreased, because at a same volume of a lithium secondary battery, more active materials can be employed. However, when the thickness of the copper foil decreases, the strength of the copper foil also decreases. After charging/discharging cycle of the battery, the thinner copper foil is easy to deform and cause wrinkles. Up until this disclosure, people like to use high tensile strength copper foil, which is difficult to deform thus causing wrinkles. However, for a conventional copper foil, when one increases the tensile strength of the foil, one basically reduces its elongation. It means that the copper foil becomes strong, but brittle.
In order for a lithium secondary battery to have a higher energy capacity, in addition to decreasing the thickness of the copper foil, a higher pressing pressure, used to condense the anode active material on the copper foil's surface, is necessary so that the lithium secondary battery can contain more anode active material. The use of conventional copper foil, of high tensile strength, means that the copper foil is not easy to deform and cause wrinkle during the charging/discharging cycle. However, this conventional copper foil, being brittle, is easily fractured at the interface of the copper foil/anode active material (carbon material) if the copper foil is subjected to higher pressure during pressing to consolidate the anode active material.
Accordingly, all these deficiencies in the prior art copper foils, especially copper foils for use as current collectors in currently available laminated type lithium secondary batteries, required the present inventors to not only recognize the deficiencies in current batteries and the components of such batteries, but also to formulate new copper foils, composites of copper foil/anode active materials, methods of production employing higher pressing pressures used to consolidate greater amounts of anode active materials, and to provide improved lithium secondary batteries of high capacity than heretofore available for the same volume battery. It is desirable to provide a rechargeable secondary battery, an electric tool, an electric vehicle, and a power storage system which can obtain an excellent battery capacity characteristic and cycle characteristics.